Process for stripping uranium from an alkyl pyrophosphoric acid

ABSTRACT

A process is described for the recovery of tetravalent uranium from wet-process phosphoric acid utilizing an alkyl pyrophosphoric acid extractant or the like. After extracting the tetravalent uranium from wet-process acid, iron is stripped from the pregnant extractant into an oxalic acid stripping solution and then the tetravalent uranium is stripped from the pregnant extractant into an oxalate stripping solution. The oxalate stripping solution is an aqueous solution of an alkali metal or ammonium oxalate. The barren extractant is recycled for contacting with fresh wet-process acid. The uranium is oxidized and then precipitated in the oxalate stripping solution. The precipitated solids are separated from the solution, and the uranium is dried to a UO 3  product.

BACKGROUND OF THE INVENTION

Minable phosphate is found in a number of places throughout the world,and in many of these deposits small quantities of uranium are foundcomplexed with the phosphate values. The large phosphate deposit incentral Florida, for example, contains from 0.01 to 0.02 weight percenturanium. This uranium is taken into solution when the phosphate isacidulated with mineral acid to produce wet-process phosphoric acid.

The early work on the recovery of uranium from wet-process acid isdescribed in a publication referred to as DOW-81 and entitled "Recoveryof Uranium from Industrial Phosphoric Acid by Solvent Extraction". Alkylpyrophosphoric acid was found to be very efficient in selectivelyextracting uranium from phosphoric acid and several flow sheetsutilizing this type of extractant are shown in U.S. Pat. No. 2,866,680.Prior processes for recovering uranium using a pyrophosphoric acidextractant include processes in which uranium is stripped from theextractant into an acidic stripping solution. It is also known to stripuranium from the extractant into an alkaline stripping solution.

It is an object of the present invention to provide an improved processfor recovering tetravalent uranium from wet-process phosphoric acidusing an extractant such as an alkyl pyrophosphoric acid and an acidicstripping solution.

A further object of the present invention is to provide an improvedprocess for recovering uranium from wet-process acid in which uranium isstripped in the tetravalent state from an extractant such as an alkylpyrophosphoric acid into an acidic stripping solution.

Still a further object of the present invention is to provide a processfor recovering tetravalent uranium from wet-process phosphoric acidusing an alkyl pyrophosphoric acid extractant and acidic strippingsolution in which the pyrophosphoric acid esters remain dissolved in theextractant solution during stripping.

Yet a further object of the present invention is to provide a processfor recovering tetravalent uranium from wet-process acid including meansfor handling any ferric iron which may be present in the pregnantextractant.

A still further object of the present invention is to provide a processfor recovering tetravalent uranium from wet-process acid which iseconomical and minimizes consumption of costly reagents.

SUMMARY OF THE INVENTION

In accordance with the present invention, uranium is stripped in thetetravalent state from an organic extractant having a high affinity fortetravalent uranium such as an alkyl pyrophosphoric acid. The strippingsolution comprises an aqueous solution of an alkali metal or ammoniumoxalate such as an aqueous solution of ammonium oxalate. During contactbetween the oxalate stripping solution and the pregnant organicextractant, uranium is stripped from the extractant into the strippingsolution.

Ferric iron is typically extracted into the organic extractant duringcontact with the wet-process phosphoric acid even when reduced acid isemployed. A portion of this ferric iron is stripped from the pregnantextractant prior to uranium stripping with an oxalic acid strippingsolution. The iron is removed from the stripping solution by heating thesolution to drive off water and then separating ferric oxalate from thesolution.

After separating the uranium stripping solution from the barren organicextractant, the uranium in the stripping solution is oxidized to thehexavalent state with an oxidizing agent such as hydrogen peroxide. Theuranium is precipitated, separated from the stripping solution and driedto yield a high-grade uranium-containing product.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic flow sheet illustrating the recovery ofuranium from wet-process phosphoric acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Wet-process phosphoric acid is obtained by the acidulation of uncalcinedphosphate rock with sulfuric acid and can contain about 20 to 55% P₂ O₅by weight, more typically about 25 to 50% P₂ O₅. Wet-process acidnormally contains about 28 to 32% P₂ O₅ by weight, but can typicallycontain about 40 to 45% P₂ O₅ if produced by the hemi-hydrate process.The wet-process acid is treated so that ferric iron in the acid isreduced to the ferrous state. Since the oxidation state of the irontends to control the oxidation state of the uranium, substantially allof the uranium will be in the reduced tetravalent state even atrelatively high ferric iron concentrations. However, to minimize ferriciron interference with extraction and stripping, the ferric iron shouldbe reduced such that the ferric iron concentration is at least about 8g/l or below, preferably at least about 4 g/l or below.

The manner in which the ferric iron is reduced is not critical. Theferric iron can be reduced using a chemical reductant such as ironmetal, silicon metal, or an iron-silicon alloy commonly referred to inthe art as a ferrosilicon alloy having a silicon content from about 5 to100% by weight, preferably about 5 to 80% by weight, and most preferablyabout 5 to 20% by weight. The ferric iron also can be reduced usingelectrolytic reduction. In the reduction step, Fe⁺³ is reduced to Fe⁺²and any U⁺⁶ to U⁺⁴. As indicated above, it is not necessary that all ofthe Fe⁺³ be reduced to the lower valence state in order to effectivelyextract the uranium from wet-process acid. Thus, the extent to which theferric iron concentration in the wet-process acid is reduced is aquestion of economics based on the overall uranium recovery process.

The reduced wet-process acid is cooled using any conventional techniquesuch as cooling water or an evaporator. The temperature of thewet-process acid is typically about 60° to 80° C. It is preferred tocool the acid to about 55° C. or below, more preferably about 30° to 50°C. Cooling the acid increases the extraction coefficient duringextraction and improves the uranium recovery process. Again, however,the extent to which the acid is cooled, if any, is a question ofeconomics based on the overall uranium recovery process.

The cooled wet-process acid is clarified to at least partially removeinorganic solids such as calcium sulfate (gypsum) and organic solidssuch as humics. There are several techniques known in the art forclarifying wet-process acid and any of these techniques can be employed.For example, a rake tank classifier or Lamella clarifier can be employedto clarify the acid. The clarification step may employ a separate solidsseparation unit. Preferably, the clarified acid is then passed through apolishing filter to reduce the solids concentration to about 20 partsper million (ppm) or less.

The tetravalent uranium in the clarified acid is extracted with anextractant having a high affinity for tetravalent uranium, preferably analkyl pyrophosphoric acid (APPA). The various APPA extractants that canbe used include those disclosed in U.S. Pat. No. 2,866,680. The APPAextractants are the reaction product of phosphoric oxide and an alcoholwhich typically has a chain length of from 4 to 20 carbon atoms,preferably from 6 to 12 carbon atoms. The reaction product is a complexmixture as indicated by "Products of the Reaction Between Alcohols andPhosphorus Pentaoxide: Identity and Extraction Efficiency of the VariousProducts", M. Zangen, Y. Marcus and E. D. Bergmann, Separation Science,(2), pp. 187-197, 1967. Methods for preparing the APPA extractants aredisclosed in U.S. Pat. Nos. 2,866,680 and 2,947,774, the disclosures ofwhich are expressly incorporated herein by reference. It is preferred toform the APPA extractants using essentially the method disclosed in U.S.Pat. No. 2,947,774.

The APPA extractant is dissolved in an essentially water-immiscibleorganic diluent. Suitable diluents include, for example, aliphatichydrocarbons, petroleum fractions low in aromatics, and chlorinatedhydrocarbons. The preferred diluents are refined high-boiling,high-flash point petroleum fractions containing between 10 and 50% byvolume naphthenes with the balance being principally aliphatic.

The APPA concentration in the extractant solution is generally about 10to 100 g/l. The concentration of APPA in the extractant solution whichis used in practice will be determined by various factors in the overallprocess such as the ferric iron concentration in the wet-process acidand the temperature to which the acid is cooled.

The extraction can be accomplished in a batch operation or in acontinuous manner concurrently or countercurrently with countercurrentflow preferred. Apparatus for accomplishing intermixing and separationof two substantially immiscible phases are well-known in the art and anyconventional apparatus can be used for this purpose. It is preferred tooperate the extraction in the aqueous continuous mode using a 3 to 8stage, preferably 5 to 8 stage, countercurrent uranium extraction unit.In general, the volume ratio of the wet-process acid to the extractantsolution during extraction should be between about 1:1 and 10:1.

After extraction, the wet-process acid is returned to the acid producerfor additional processing to make "merchant acid", which is either soldor used to manufacture a variety of products, chiefly fertilizers.Preferably, any extractant entrained in the wet-process acid duringextraction is removed from the wet-process acid before the acid isreturned to the acid producer. The entrained extractant can be removedby any conventional technique including the use of one or more flotationcells.

The extractant is stripped of its iron content by contacting theextractant with an oxalic acid stripping solution. In general, the pH ofthe oxalic acid stripping solution should be about 1.5 to 2, preferablyabout 1.5 to 1.8. A 1 M oxalic acid solution has a pH of 1.5. Thetemperature in this stripping stage will normally be about the same asthe temperature to which the acid is cooled prior to extraction.

The iron stripping can be accomplished in a batch operation or in acontinuous manner concurrently or countercurrently with countercurrentflow preferred. It is preferred to operate the iron stripping in theorganic continuous mode using a 2 to 5 stage, preferably 3 stage,countercurrent stripping unit. In general, the volume ratio of theoxalic acid stripping solution to the extractant solution duringstripping should be between about 1:10 and 1:1, preferably about 1:4.

The iron is removed from the oxalic acid stripping solution by heatingthe solution to drive off water. The ferric oxalate precipitate is thenseparated from the stripping solution in a solids separator such as acentrifuge or filter. The oxalic acid stripping solution is recycled forcontact with fresh pregnant extractant. Make-up stripping solution isadded to the recycled stream.

In the next step of the process, the extractant is stripped of itsuranium content by contacting the extractant with an oxalate strippingsolution. The oxalate stripping solution is an aqueous solution of analkali metal or ammonium oxalate. The preferred stripping solution is anammonium oxalate solution. The terminology "oxalate" also is intended toencompass bioxalate.

In general, the pH of the oxalate stripping solution should be aboveabout 3.5, preferably above about 5, and more preferably about 5 to 5.5.For example, if an ammonium oxalate solution is used, it shouldpreferably be about 0.5 to 3 M, more preferably about 1 M. Thetemperature in this stripping stage will normally be about the same asthe temperature to which the acid is cooled prior to extraction.

The uranium stripping can be accomplished in a batch operation or acontinuous manner concurrently or countercurrently with countercurrentflow preferred. It is preferred to operate the uranium stripping in theorganic continuous mode using a 2 to 5 stage, preferably 3 stage,countercurrent uranium stripping unit. In general, the volume ratio ofthe oxalate stripping solution to the extractant solution duringstripping should be between about 1:10 and 1:1, preferably between about1:2 and 1:5.

The barren extractant is recycled for contact with fresh wet-processacid. Prior to contact with the acid, however, a portion of the recycledbarren extractant is removed as a bleed stream. The bleed stream ispassed through a distillation column or the like or scrubbed with analkaline solution to separate the diluent contained in the bleed streamfrom the APPA and decomposition products such as orthoesters. Thediluent is then combined with fresh APPA extractant and fed back intothe recycled barren extractant stream.

The volume of the bleed stream, which is typically about 5 to 15% of therecycled barren organic extractant stream, is based on the quantity offresh APPA required to be added to keep the APPA concentration in theextraction solution approximately the same. The APPA concentration ofthe fresh extractant is typically about 20 to 1000 g/l, preferably about400 to 600 g/l. The volume of fresh extractant fed back, after dilutionwith recovered diluent, into the recycled barren extractant stream willbe substantially the same as the volume of extractant solution removedin the bleed stream.

The recycled barren extractant stream may, if desired, be acidulatedwith mineral acid such as phosphoric acid prior to being recontactedwith wet-process acid in extraction. The acidulation converts the APPAextractant from its salt form (e.g., ammonium form) to its acid form.Otherwise, the extractant will be acidified during the first stages ofcontact with the wet-process acid.

The uranium in the oxalate stripping solution is oxidized and theuranium converted from the tetravalent state to the hexavalent state inthe stripping solution. The preferred oxidizing agent is hydrogenperoxide; however, other oxidizing agents such as sodium chlorate,oxygen, or air could be employed. The hydrogen peroxide is preferablyadded to the stripping solution as an about 25 to 50% by volume aqueoussolution. The amount of oxidizing agent added should be in excess of thestoichiometric amount required to oxidize the uranium content of thestripping solution to the +6 state (UO₂ ⁺⁺).

In the next step of the process, the oxalate stripping solution istreated to cause the uranium to precipitate as ammonium diuranate (ADU).The pH of the stripping solution prior to the precipitation step isabout 3 to 5, preferably about 5. The pH is adjusted during theprecipitation step to at least about 9. This is accomplished, forexample, by adding recycled ammonia to the stripping solution. Theparticular pH involved will depend on the ammonia concentration.

The oxalate stripping solution is fed to a conventional solids separatorsuch as a centrifuge or filter. The precipitate is separated from theoxalate stripping solution in the solids separator. The oxalatestripping solution is recycled, and the uranium precipitate dried in aconventional dryer which is preferably indirectly fired.

The recycled oxalate stripping solution is passed through a distillationcolumn or the like to separate ammonia from the stripping solution. Theammonia is recycled to the precipitation unit to adjust the pH. The pHof the oxalate stripping solution leaving the distillation column or thelike is about 6 to 8, preferably about 6.5.

Prior to contact with the pregnant extractant, a portion of the recycledoxalate stripping solution is removed as a bleed stream. Subsequently,make-up oxalic acid solution and any necessary make-up ammonia is addedto the recycled stream in sufficient quantity and concentration toadjust the pH of the solution to preferably about 5 to 5.5 and tomaintain the concentration of the stripping solution in the desiredrange.

In order to facilitate an easier understanding of the uranium recoverystages of the present invention, a flow sheet illustrating the processis provided in the FIGURE. Wet-process phosphoric acid is introducedinto ferric iron reduction unit 10 and the ferric iron concentrationreduced. The reduced wet-process acid is introduced via line 14 intocooling unit 16 in which the acid is cooled. After cooling, the acid isfed via line 18 to clarification unit 20. The solids separated inclarification unit 20 are fed via line 24 to solids separator 26. In theseparator 26, residual acid is separated from the solids and recycled toextraction via line 28. The separated solids are fed to disposal vialine 30.

The clarified acid is introduced via line 34 into a countercurrenturanium extraction unit 36, while an APPA extractant solution isintroduced into the unit via recycle line 38. After extraction, theraffinate acid is fed to extractant removal unit 40 via line 44. Theraffinate acid is returned to the acid producer via line 46 to beevaporated into "merchant acid". The extractant removed from theraffinate acid in extractant removal unit 40 is recycled to extractionvia line 48.

The extractant, now rich in uranium, is fed via line 50 to ironstripping unit 54, while an oxalic acid stripping solution is introducedinto the unit via recycle line 56. After being stripped of its ironcontent, the extractant from iron stripping unit 54 is fed to uraniumstripping unit 58 via line 60 and is then recycled to uranium extractionunit 36 via recycle line 38.

A portion of the recycled barren extractant is removed from recycle line38 via bleed line 64 and fed to distillation unit 66. The overhead fromthe distillation unit 66, principally diluent, is fed via line 68 toextractant make-up tank 70, while the bottoms from the distillation unit66, principally APPA extractant and hydrolysis products, are fed todiscard via line 74. Fresh APPA is introduced into extractant make-uptank 70 via line 76 and the resulting fresh extractant solutionintroduced into recycle line 38 via line 78.

The oxalic acid stripping solution from iron stripping unit 54 is fed toiron removal unit 80 via line 84. The iron is removed from the strippingsolution by heating the solution to drive off water and then separatingferric oxalate solids via line 86. Stripping solution makeup is added torecycle line 56 via line 88.

The ammonium oxalate stripping solution is introduced into uraniumstripping unit 58 via recycle line 90. The ammonium oxalate strippingsolution from uranium stripping unit 58 is fed via line 94 to oxidationunit 96 and then introduced into ADU (ammonium diuranate) precipitationunit 98 via line 100, while ammonia is introduced into the unit viarecycle line 104. The ammonium oxalate stripping solution fromprecipitator 98 is fed via line 106 to solids separator 108. Theammonium oxalate stripping solution from solids separator 108 is fed todistillation unit 110 via line 112. A portion of the recycled strippingsolution is removed from recycle line 90 via bleed line 114 and fed todiscard or recovery. Oxalic acid solution make-up is fed to thestripping solution recycled to contact fresh pregnant extractant vialine 116. The ADU solids separated in solids separator 108 areintroduced into ADU dryer unit 118 via line 120. The ADU solids aredried in ADU drier unit 118 to yield a high-grade UO₃ product.

To facilitate a better understanding of the advantages and operation ofthe present invention, the following examples are provided.

EXAMPLE 1

A barren extractant solution containing 30 g/l isodecyl pyrophosphoricacid in a kerosene diluent was contacted in a series of three sequentialbatch shakeouts with wet-process phosphoric acid. The first contact ofthe extractant solution with the wet-process acid was at an aqueous toorganic ratio of 3:1, the second contact was at an aqueous to organicratio of 5:1 and the third contact at an aqueous to organic ratio of7:1. Each of the contacts were standard batch shakeouts at 55° C. of 5minutes' duration. Fresh wet-process phosphoric acid was used in eachcontact. The wet-process acid contained 165 mg/l total uranium, 3 g/lferric iron and 29% P₂ O₅ by weight. The uranium concentrations of theraffinate acids from the third, second and first contacts were 0.115g/l, 0.082 g/l and 0.032 g/l, respectively. The pregnant extractantcontained 1.175 g/l total uranium and 0.9 g/l ferric iron.

The pregnant extractant was contacted in a series of two sequentialbatch shakeouts with a 1 molar aqueous solution of oxalic acid having apH of 1.8. The first contact of the pregnant extractant with the oxalicacid solution was at an organic to aqueous ratio of 10:1 and the secondcontact was at an organic to aqueous ratio of 5:1. Each of the contactswere standard batch shakeouts at 65° C. of 5 minutes' duration. Freshoxalic acid solution was used in each contact. The ferric ironconcentration of the pregnant extractant after contact with the oxalicacid solution was 0.15 g/l.

The pregnant extractant was then contacted in a series of threesequential batch shakeouts with a 1 molar aqueous solution of ammoniumoxalate having a pH of 3.8. The contacts were all at an organic toaqueous ratio of 4:1. Each of the contacts were standard batch shakeoutsat 65° C. of 5 minutes' duration. Fresh ammonium oxalate solution wasused in each contact. The uranium concentrations of the ammonium oxalatesolutions from the first, second and third contacts of the pregnantextractant with the ammonium oxalate solutions were 3.224 g/l, 0.924 g/land 0.215 g/l, respectively. The uranium concentration of the barrenextractant after the final (third) contact with the ammonium oxalatesolutions was 0.066 g/l.

EXAMPLE 2

The process described in Example 1 was repeated except that thewet-process acid contained 179 mg/l total uranium. The uraniumconcentrations of the raffinate acids from the third, second and firstcontacts were 0.171 g/l, 0.133 g/l and 0.058 g/l, respectively. Thepregnant extractant contained 0.9 g/l total uranium and 1.0 g/l ferriciron. The ferric iron concentration of the pregnant extractant aftercontact with the oxalic acid solution was 0.15 g/l. The uraniumconcentrations of the ammonium oxalate solutions from the first, secondand third contacts of the pregnant extractant with the ammonium oxalatesolutions were 0.3813 g/l, 0.439 g/l and 0.061 g/l, respectively. Theuranium concentration of the barren extractant after the final (third)contact with the ammonium oxalate solutions was 0.026 g/l.

EXAMPLE 3

A pregnant extractant solution containing 30 g/l isodecyl pyrophosphoricacid in a kerosene diluent, a total uranium concentration of 1.113 g/land a ferric iron concentration of 0.7 g/l was contacted in a series offour sequential batch shakeouts with a 1 molar aqueous solution ofoxalic acid having a pH of 1.7. The first contact of the pregnantextractant with the oxalic acid solution was at an organic to aqueousratio of 2:1, the second contact was at an organic to aqueous ratio of4:1, the third contact was at an organic to aqueous ratio of 6:1 and thefourth contact was at an organic to aqueous ratio of 10:1. Each of thecontacts were standard batch shakeouts at 50° C. of 5 minutes' duration.Fresh oxalic acid solution was used in each contact. The ferric ironconcentration of the pregnant extractant after contact with the oxalicacid solution was 0.2 g/l.

The pregnant extractant was then contacted in a series of threesequential batch shakeouts with a 1 molar aqueous solution of ammoniumoxalate having a pH of 5. The contacts were all at an organic to aqueousratio of 4:1. Each of the contacts were standard batch shakeouts at 65°C. of 5 minutes' duration. Fresh ammonium oxalate solution was used ineach contact. The uranium concentrations of the ammonium oxalatesolutions from the first, second, third and fourth contacts of thepregnant extractant with the ammonium oxalate solutions were 3.774 g/l,1.82 g/l, 1.342 g/l and 0.986 g/l, respectively. The uraniumconcentration of the barren extractant after the final (fourth) contactwith the ammonium oxalate solutions was 0.116 g/l.

As will be readily understood by those of ordinary skill in the art,minor modifications may be made in the process described above withoutin any way departing from the spirit and scope of the invention.Accordingly, it is understood that the invention will not be limited tothe exact details disclosed hereinbefore, but will be defined inaccordance with the appended claims.

We claim:
 1. A process for stripping uranium from a pregnant organicextractant comprising an extractant having a high affinity fortetravalent uranium dissolved in a substantially water-immiscibleorganic diluent, said organic extractant containing tetravalent uranium,comprising stripping said organic extractant with a stripping solutioncomprising an aqueous solution of an alkali metal or ammonium oxalate tostrip tetravalent uranium from said organic extractant into saidstripping solution, and separating said organic extractant from saidstripping solution containing stripped tetravalent uranium.
 2. Theprocess of claim 1 in which said extractant is an alkyl pyrophosphoricacid which is a reaction product of phosphoric oxide and an alcoholcontaining 4 to 20 carbon atoms.
 3. The process of claim 2 wherein saidalcohol contains 6 to 12 carbon atoms.
 4. The process of claim 2 inwhich said organic extractant contains about 10 to 100 g/l of said alkylpyrophosphoric acid.
 5. The process of claim 1 in which said strippingsolution is an aqueous ammonium oxalate solution.
 6. The process ofclaim 1 in which said stripping solution has a pH of above about
 5. 7.The process of claim 6 in which said pH is about 5 to 5.5.
 8. A processfor stripping uranium from an organic extractant comprising an alkylpyrophosphoric acid dissolved in an essentially water-immiscible organicdiluent, said organic extractant containing tetravalent uranium andferric iron, stripping said organic extractant with an oxalic acidstripping solution to strip ferric iron from said organic extractantinto said oxalic acid stripping solution, and stripping said organicextractant with an alkali metal or ammonium oxalate stripping solutionto strip tetravalent uranium from said organic extractant into saidoxalate stripping solution.
 9. The process of claim 8 in which saidalkyl pyrophosphoric acid is a reaction product of phosphoric oxide andan alcohol containing 4 to 20 carbon atoms.
 10. The process of claim 9in which said alcohol contains 6 to 12 carbon atoms.
 11. The process ofclaim 8 in which said organic extractant contains about 10 to 100 g/l ofsaid alkyl pyrophosphoric acid.
 12. The process of claim 8 in which saidoxalate stripping solution is an aqueous ammonium oxalate solution. 13.The process of claim 8 in which the pH of said oxalic acid strippingsolution is about 1.5 to
 2. 14. The process of claim 8 in which the pHof said oxalate stripping solution is about 5 to 5.5.
 15. A process forrecovering uranium from wet-process phosphoric acid comprisingcontacting wet-process phosphoric acid containing tetravalent uraniumwith an organic extractant to extract said tetravalent uranium from saidwet-process phosphoric acid, said organic extractant comprising an alkylpyrophosphoric acid dissolved in an essentially water-immiscible organicdiluent, separating said organic extractant containing said tetravalenturanium from said wet-process phosphoric acid, contacting said pregnantorganic extractant with a first stripping solution comprising an aqueoussolution of oxalic acid to strip iron from said pregnant organicextractant into said first stripping solution, contacting said pregnantorganic extractant with a second stripping solution comprising anaqueous solution of an alkali metal or ammonium oxalate to strip uraniumfrom said pregnant organic extractant into said stripping solution, andrecycling barren organic extractant for contacting with freshwet-process phosphoric acid.
 16. The process of claim 15 in which aportion of said recycled barren organic extractant is treated toseparate diluent from the remainder of said organic extractant, freshalkyl pyrophosphoric acid is added to said separated diluent to formfresh organic extractant, and said fresh organic extractant is added tosaid recycled barren organic extractant.
 17. The process of claim 16 inwhich said diluent is separated by distillation.
 18. The process ofclaim 15 in which said recycled barren organic extractant isreacidulated with a mineral acid.
 19. The process of claim 15 in whichsaid stripped uranium is precipitated in said oxalate strippingsolution, said precipitate is separated from said stripping solution,and said stripping solution is recycled for contact with pregnantorganic extractant.
 20. The process of claim 19 in said saidprecipitated uranium is dried to form a final UO₃ product.
 21. A processfor recovering uranium from wet-process phosphoric acid comprisingcontacting wet-process phosphoric acid containing tetravalent uraniumwith an organic extractant to extract said tetravalent uranium from saidwet-process phosphoric acid, said organic extractant comprising an alkylpyrophosphoric acid dissolved in an essentially water-immiscible organicdiluent, contacting said pregnant organic extractant with a strippingsolution comprising an aqueous solution of an alkali metal or ammoniumoxalate to strip uranium from said pregnant organic extractant into saidstripping solution, separating said organic extractant containing saidalkyl pyrophosphoric acid from said stripping solution containing saidstripped uranium, recycling barren organic extractant for contactingwith fresh wet-process phosphoric acid, treating a portion of saidrecycled barren organic extractant to separate diluent from theremainder of said organic extractant, adding fresh alkyl pyrophosphoricacid to said separated diluent to form fresh organic extractant, addingsaid fresh organic extractant to said recycled barren organicextractant, separating said uranium from said stripping solution byprecipitation, and recycling said stripping solution for contacting withfresh organic extractant.
 22. The process of claim 21 in which saidpregnant organic extractant is stripped with an oxalic acid strippingsolution to strip ferric iron from said organic extractant.
 23. Theprocess of claim 21 in which said precipitated uranium is dried to forma final UO₃ product.